Remember, each time you do this, your local sources will "flip". However, when you flip from old-branch to new-branch, you will probably want the changes from old-branch to propagate into new-branch - here's how:

Submit new-branch after reviews & approvals

Because new-branch builds on old-branch, this would only happen after old-branch has been submitted (see above).

TBD: Because time may have elapsed since old-branch was submitted, and cleanup steps may or may not have been done, it would be nice if we could show here how to "check" and confirm that new-branch has been properly rebased and is independent of the no-longer-pending old-branch.